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Would it be safe to assume that

1) The platters are as near to "optically flat" as can be? Let's keep
it to hard disks from the last 15 or so years. (Maybe the beasts from
the '80s were rougher?)

2) That the mechanical tolerances are in the 0.1 mil range?

Would it be reasonable to transform a hard disk into a lapping
machine? Say you slap a fine grit 3M lapping paper on the platter,
somehow, and lap away?

You could even hijack the actuator/voice coil into a system that moves
the work piece back and forth.

Opinions, please. Blast away!

TIA

Re: hard disk questions...

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Would undulations in the lapping paper & the way it is applied WAY outweigh
the "optical flatness" of the disk surface?


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Re: hard disk questions...
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Probably. One grain of dust under there and it's garbage... But I
remember years ago I had to lap fiber optic terminations. I just
slapped a piece of paper onto the block and lapped away. Then again,
the size of the contact area of the fiber relative to bumpiness in the
setup was small.

I'm guessing the larger the piece being lapped, the more important
overall flatness of the setup becomes.

Re: hard disk questions...
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You really don't want to try lapping at 7200 rpm.  Also the drive won't
start if it finds anything seriously wrong with itself.

You might be able to take an old 10 MB XT drive and do something with that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: hard disk questions...
On Jul 4, 10:54A0%pm, Phil Hobbs
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Oh heavens no. I'll strip out all the electronics and drive the
platters much slower.
Is my idea at least not entirely ridiculous?

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Easier said than done! Hard to find these days. But you never know.
The last really big hard drive (1GB ESDI full height 5.25) I found was
stripped for its magnets, I tossed the carcass out.
Maybe the local hacker space has some oldies.

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outweigh
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The motor won't have much torque for a heavy-duty job like lapping, so
you'll have to be gentle with it.

The bearings won't last long, since they're designed to work in a very
clean environment.  Also many HDs have glass platters, which would be
very dangerous to use that way.  I think only IBM/Hitachi drives use
glass, but I might well be wrong.

However, if you have a dead hard drive and are interested in learning
how to drive brushless DC motors, go for it.  (The knowledge will last a
lot longer than the lapping tool, and it might be a fun project.)


Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: hard disk questions...




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It is fun to shoot HDDs. The 9x19 lead core usually can't make it all
way through a 2-platter. The platters are made of a pretty strong alloy.
Although I have heard of a glass platter HDDs intended for laptops, I am
yet to shoot one.

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The VHS rotating head assembly and a laser printer rotating mirror drive
are very similar to a HDD motor. They might even use the same ICs.

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Yea, long while ago I used to repair such things.

VLV

Re: hard disk questions...

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Not enough torque.

Re: hard disk questions...
On Jul 4, 10:58A0%pm, " snipped-for-privacy@att.bizzzzzzzzzzzz"
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Eek, just how much pressure do you think I'll put on the work piece?
It can take a week for all I care.

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How much torque do you think it takes to drive a polished, optically flat,
disk through still air with nothing touching it?  Now you're touching it.

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The torque requirement is dominated by head stiction rather than
continuous rotation.  The motor has to have enough torque to start up
with the heads sitting on the platters.  Of course the available torque
declines as the motor speeds up.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: hard disk questions...
On Tue, 05 Jul 2011 00:39:34 -0400, Phil Hobbs

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Modern drives unload the heads on power down.  There is a ramp at the end of
the head travel where they're parked.  Haven't had stiction issues for
decades.

I would think torque would be more or less constant over RPM for that sort of
motor.

Re: hard disk questions...
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That isn't the case for all drives.  The IBM/Hitachi ones (those that
I'm most familiar with) use a textured landing zone on the inner edge of
the disc surface, which is cheaper and also avoids letting particles
under the slider.  Stiction issues still come up because the lube
sometimes migrates around, despite being chemically anchored to the platter.

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Once you get past the stiction, it's like a fan motor, where the
required torque goes like the square of the rotation speed.  So they
need some grunt to get the platters going, but then they speed up really
fast.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
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Re: hard disk questions...
On Tue, 05 Jul 2011 01:12:13 -0400, Phil Hobbs

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Old ones.  IBM switched with their glass disks.  Dunno what Hitachi has done
since.  Never had one.

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They're not a servo motor?

Re: hard disk questions...
On Jul 4, 11:32A0%pm, " snipped-for-privacy@att.bizzzzzzzzzzzz"
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About as much as it took when it was a hard disk? AFAIK, they don't
operate in a vacuum.

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Of course not.  The point being that it doesn't take much to move a polished
disk through still air.  It takes quite a bit, once you drop a gouge on it.
;-)

Re: hard disk questions...
And if you're bored tonight, search eBay for st-506 drive or MFM
drive...

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Nowadays they have measured the characteristic vibrations at differing
speeds and know how to compensate. the whole disk acts like a moving
wave.

Hardy

Re: hard disk questions...
On Thu, 7 Jul 2011 20:27:30 -0700 (PDT), HardySpicer

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  Yeah, and NONE of the slop in ANY hard drive spindle motor is going to
affect lapping processes, nor will the platter surface quality.
Particularly if all the dope is gonna do is place paper on top of it.

  Also, the magnetic coating IS going to wear off even before any of the
work item does, so what you will be working with is a hard, high quality
Aluminum platter that probably has the best surface quality of anything
Aluminum you ever saw short of an optical flat, which is really glass
with an aluminized surface.  Hardly a 'finished' surface, which is what
the platter will be after the rouge<sp> takes the mag layer off, which
will be almost immediate.

 They would be fine for lapping purposes.  They, in fact, have the most
axially stable motors the industry makes short of some laser gear. They
were top dog for decades in that realm though.  It would be interesting
to see the numbers on the little 1.5 and 1.2 inch 10k and 15k rpm drive
motors in the SAS channel. They are made to spin at those rates
indefinitely and hot.  Pretty good motor engineering if ya ask me.

  Think of how much force the motor would have to produce if we were
still using 5.25 inch full height form factor drives.

 I still have such a motor, and it will even operate as a generator, so I
was thinking about rigging up a bike light energizer with it.  It has
rings on it which were the spacers that were between each platter, which
were a simple compression stack.  It was fun to tear that HS down. I
posted pictures of it in ABSE a long time ago, but that was years in the
past.  It has HUGE rare earth magnets in it and the heads were moved by a
linear motor on a bearing suspended gantry.  It was one of the coolest
drive guts I have ever seen.  The heads were absolutely linear travel.
8 platters for 16 heads.  A TRUE 16 head drive.  It would be cool to see
what a perpendicular recording head array and a similar drive would hold,
but they are to vulnerable to failure when there are so many parts.

  So, we make SMALL form factor drives that do not hold as much as a
multi-platter array drive, but will have a much higher MTBF.  If you want
more space, simply ad more drives or get an array.  That way, lost data
can be rebuilt onto the failed drive location after it gets replaced.

 The heads only have to move a little over a half inch too.  A big drive
means big head arm swings, and HUGE latencies as a result.

  Modern 1.5 inch 15k rpm drives are the state of the art, truly.

 Years ago, I used to delve into the more expensive realm of SCSI
business class HDs.  They were not that much more then.

  But now, getting into SAS is NOT a cheap proposition, but the drives
are SWEET! They get so much onto such a small space! Sounds scary,
actually. Seems like you could read it optically in an electron
microscope.  :-)

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Nonsense.  The resonances are of course managed carefully, but if you
had high order resonances excited in any significant amplitude, you'd
never make the file work.

The slider air bearing is extremely stiff--the flying height is on the
order of 100 angstroms, because in magnetostatics you can't resolve bits
smaller than the flying height.  (Vertical recording helps some, but not
really dramatically.)  That's just a consequence of Laplace's equation.

There's a _lot_ of physics in the head-disk interface.

Cheers

Phil Hobbs




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Dr Philip C D Hobbs
Principal Consultant
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